1. Field of the Invention
This invention relates to an intensity dividing device for use in division of radiation.
2. Discussion of Prior Art
Radiation intensity dividing devices are known, such as for example optical fibre Y-junctions. Y-junctions may be symmetrical, for division of one input beam into two substantially equal intensity beams. Such devices are discussed by Z. Weissman, A. Hardy and E. Marom in "Mode-Dependent Radiation Loss in Y Junctions and Directional Couplers", IEEE Journal of Quantum Electronics, Vol 25, No 6 (1989) pp 1200-1208. Active symmetric Y-junctions which employ electro-optic effects to achieve asymmetric splitting are also known. An example is described by H. Sasaki and I. Anderson in "Theoretical and Experimental Studies on Active Y-Junctions in Optical Waveguides", IEEE Journal of Quantum Electronics, Vol QE-14, No 11 (1978) pp 883-892. However, symmetrical Y-junctions, both active and passive, suffer from high losses, particularly for split angles greater than a few degrees.
Asymmetric Y-junctions capable of dividing an input beam into two beams of differing intensities are also known. One such device is described by K. Shirafuji and S. Karazono in "Transmission Characteristics of Optical Asymmetric Y Junction with a Gap Region", Journal of Lightwave Technology, Vol 9, No 4 (1991) pp 426-429. It is considerably more efficient than more conventional Y-junctions since it uses total internal reflection to redirect radiation to one of the two output parts. Radiation reaches the other output port by coupling across a gap. This radiation is not deviated from the input direction of propagation. The power splitting ratio is determined by the width of the gap.
All Y-junctions, however, suffer from the disadvantage that they can only provide two way splitting. Therefore, to achieve higher order splitting Y-junctions are used in series, thus multiplying the losses incurred at each stage.
Many other forms of intensity dividing device are also known. In International Patent Application No. PCT/US89/00190, published under International Publication No. WO 89/06813 E. Kapon describes optical waveguide junctions. One incorporates a single input waveguide with four single mode output waveguides of differing widths and/or differing refractive indices, radiating from an end. The output waveguides are therefore characterised by different propagation constants. For a given input wavelength, different modes of the input waveguide will couple to different output waveguides, as a result of the different propagation constants. However, this is an inefficient device with high transmission losses, since energy from each mode will in general enter each output waveguide but will be lost from those with unfavourable propagation constants.
An alternative device described by E. Kapon incorporates four single mode input waveguides of differing widths and/or refractive indices, converging into an area from which three single mode output waveguides radiate. The modes excited in the common area are dependent on which input waveguides are providing radiation beams. The output waveguides operate as described for the single input devices. These devices enable radiation to be divided according to the ratio of excitation of modes in the waveguides feeding the output waveguides. However, as previously stated they are highly inefficient.
In U.S. Pat. No. 4,693,546 J. P. Lorenzo and R. A. Soref describe a "Guided Wave Optical Power Divider". It is in the form of an X-junction. Two input waveguides converge on an input end of a crossover region and two output waveguides diverge from an output end of the region. The input and output waveguides are single mode and of width W. The crossover region supports two modes, one odd and one even, and is of width 2 W. The device is formed from crystalline silicon and the crossover region is doped. In an undoped device radiation passes through the crossover region substantially undeviated, and enters the first output waveguide. In a doped device waveguide modes are perturbed and a fraction of the input radiation is deviated whilst passing through the crossover region and enters the second output waveguide. The proportion of light deviated is determined by the level of doping and may be in the range 10 to 20%. Lorenzo and Soref do not mention the losses suffered in these devices, other than to state that the addition of dopants increases absorption by a small amount.
In order to produce more complex devices a number of the X-junctions are combined. They may, for instance, be used to form a predetermined optical signal distribution network. However, such networks are quite complex, for even a modest number of inputs and outputs.
Another form of intensity dividing device is described by A. Mahapatra and J. M. Connors in European Patent Application 88108258.0, Publication No 0 301 194. The devices described incorporate one or more input channels, and a number of output channels, provided on opposite sides of a planar waveguide. Essentially, radiation input to the planar waveguide fans out and is thus incident on apertures of the output waveguides. The patent application describes how the devices may be constructed to improve uniformity of coupling. That is to obtain substantially equal intensity in each of the output waveguides. These devices will be inherently inefficient, since much of the radiation input to the planar waveguide will be incident on portions of waveguide wall between output waveguide apertures. As a result it will either be absorbed or reflected back to interfere with radiation in the planar waveguide.
Yet another form of device is described by T. P. Young and I.R. Croston in UK Patent Application 2 215 482A, which is entitled "Optical In-line filter". It incorporates a first multimode waveguide whose output end is coupled to an open end of a second, narrower waveguide. The coupling and the length of the first waveguide being such that light entering the first waveguide undergoes interference between at least two of its modes. This causes light from the first waveguide of a predetermined waveband, or wavebands, only to enter the second waveguide. Light of other wavebands is provided with alternative means by which it may leave the first waveguide. Thus the device is capable of separating one waveband (or a set of harmonic wavebands) from others using modal dispersion. It thereby divides the intensity of the input radiation, but that is purely a by-product of the purpose of the device, the proportions of power in the various output beams are determined by the proportions of different wavelengths in the input beam. There is no means by which the contribution from one wavelength may be divided.
A further form of waveguide intensity dividing device using modal dispersion is described in UK Patent No. 1 525 492 entitled "Self Imaging System Using a Waveguide". This describes many different devices using multimode waveguides supporting at least fifty modes. FIG. 33(a) illustrates a device for dividing a single input image into a number of identical images of differing intensities. An input waveguide feeds an image into a first rectangular waveguide, of width .omega..sub.z and length L. A large number of modes are excited in the first rectangular waveguide, modal dispersion occurs and after a length L five images, each substantially one fifth the intensity of the input image, are produced. Three of the five images pass to a second rectangular waveguide of width .omega..sub.z ' and length L', whilst the remaining two pass to a third rectangular waveguide of width .omega..sub.z " and length L". Modal dispersion also occurs in the second and third rectangular waveguides. Thus after a length L' the three images are recombined. Likewise after a length L" the two images are recombined. The second and third rectangular waveguides each feed an output waveguide of like dimensions to the input waveguide. The images entering these waveguides have intensities in the ratio 3:2.
The device described in UK Patent No. 1,525,492 is more efficient than previously described prior art devices. However it suffers from a number of disadvantages. It requires waveguides capable of supporting a large number of modes, and these are difficult to produce. In addition it is fairly complex, and inconveniently long for many applications.